Added elastic band dimensions

docs/openflexure microscope.docubricks.xml

@@ -110,7 +110,7 @@
     </manufacturing_instruction>
   </physical_part>
   <physical_part id="1798060650">
-    <description>Elastic bands, qty:3</description>
+    <description>Elastic bands, No. 32, Qty:3</description>
     <supplier />
     <supplier_part_num />
     <manufacturer_part_num />
@@ -120,7 +120,7 @@
     <media />
     <manufacturing_instruction>
       <step>
-        <description>The elastic bands are important to provide a return force on the nut that is pulled upwards by the screw.  I use number ??? bands, which are about 3mm wide and about ???mm in circumference.  It would also be possible to use shorter bands and wrap them round the foot fewer times.  If only thin elastic bands are available, you may need to use multiple bands or fold bands in half to get enough force.</description>
+        <description>The elastic bands are important to provide a return force on the nut that is pulled upwards by the screw.  I use number 32 bands, which are about 3mm wide and about 150mm in circumference (i.e. 75mm long when measured as a loop.  It would also be possible to use shorter bands and wrap them round the foot fewer times.  If only thin elastic bands are available, you may need to use multiple bands or fold bands in half to get enough force.</description>
         <media />
       </step>
     </manufacturing_instruction>